This invention relates generally to plasma chambers used for fabricating semiconductor devices. More specifically, this invention relates to apparatus and methods for monitoring and controlling the electrical states at a workpiece mounted to one or more chamber electrodes.
Fabrication of semiconductor devices often utilizes plasma processing equipment for etch and deposition steps that are needed in order to create transistors on the surface of silicon wafers or other workpieces. Usually, these plasma processing reactors have conductive walls and employ a support pedestal upon which the workpiece is placed during the process. Typically, the pedestal includes a conductive body that is connected to a radio frequency RF power supply. During most processing steps, the power supply is activated, with the conductive body functioning as a cathode electrode and the walls functioning as an anode electrode. In this manner, the electrical states at the workpiece are varied during processing.
For example, the body of the plasma is positively charged with respect to the average DC potential on the cathode and anode electrodes. The DC voltage at the cathode is referred to as the xe2x80x9ccathode DC biasxe2x80x9d and changes the electrical states at the workpiece so that ions from the plasma accelerate toward and bombard the workpiece to promote chemical or physical reactions desired for the semiconductor fabrication process. The cathode DC bias also has an influence on the ability to electrostatically clamp the workpiece to the pedestal. Typically, a DC voltage is applied to the pedestal to vary the electrical states at the workpiece in order to create an electrostatic force between the workpiece and the surface of the pedestal. Presence of DC bias voltage due to the applied RF signal will superimpose on the DC voltage supplied for the electrostatic force. The superimposition of the two voltages alters the electrical states at the workpiece by varying the electrostatic force by either reducing or increasing the same, depending on the polarity of the DC supply. Since it is desirable to maintain a constant electrostatic force during processing, accurate knowledge of the electrical states, such as DC bias voltage, at the workpiece is desired. As a result, there are many prior art attempts to determine and/or control the electrical states at a workpiece.
Traditionally, the electrical states at the workpiece were determined by preventing transmission losses in the signal path between a voltage source and the cathode electrode. According to one traditional manner, RF power supplied to a cathode was determined by controlling the resistivity of the load connected to the RF generator. To that end, RF generator is coupled to the process chamber through a matching network. The matching network transforms the complex load of plasma and chamber so they appear to the generator as a purely 50 xcexa9 resistive load. The 50 xcexa9 resistive load is believed to maximize the power delivered to the cathode; hence, the power measured at the generator output was believed to match the power level delivered at the cathode. However, the actual power delivered to the electrode often differed from the power generated by the generator.
Another technique for determining the electrical states at a workpiece involved determining the set point of the electrode DC bias. This is achieved by employing an RF peak detecting circuit coupled to the electrode. The peak detecting circuit is included with a circuit for controlling the DC voltage applied to the chuck. The control circuit supplies an unamplified replica of a DC voltage derived by the peak detecting circuit to the chuck DC power supply source via a DC circuit including only passive elements so the level of the DC voltage applied to the chuck varies in response to variations in the peak amplitude of the RF voltage.
Another method of controlling cathode DC bias in a plasma chamber employs a dielectric shield. The dielectric shield is positioned between the plasma and a selected portion of the electrically grounded components of the chamber, such as the electrically grounded chamber wall. The cathode DC bias is adjusted by controlling one or more of the following parameters: (1) the surface area of the chamber wall or other grounded components which is blocked by the dielectric shield; (2) the thickness of the dielectric; (3) the gap between the shield and the chamber wall; and (4) the dielectric constant of the dielectric material.
As stated by Patrick et al. in Application of Direct Bias Control in High-Density Inductively Coupled Plasma Etching Equipment, J. Vac. Sci. Technol. A 18(2), March/April 2000 it is difficult to provide reproducible DC bias voltages on successively processed wafers, because, any variation in match resistance leads directly to differences in power delivered to the load for the given RF power supply setpoint. This may be attributed to, inter alia, to variations in RF power supply and match network resistance, as well as the presence of stray capacitance. Patrick et al. advocate use of a peak voltage sensor mounted immediately below the chuck in a feedback loop to the RF generator to accurately determine the electrode DC bias. Controlling the power delivery in this manner facilitates compensating for the effects of power losses in the RF circuit between the generator and the chuck.
A drawback with these prior art techniques that employed sensors was that the same often provide inaccurate measurements of the power level at the electrode. The accuracy of the sensor measurements are degraded at high electrode voltages due to the presence of large signal noise. In addition, accurately calibrating a sensor placed near the workpiece electrode can also be problematic due to the limited access area available.
As a result, there is a need to provide an improved technique for determining the electrical states at a workpiece during processing.
A method and apparatus is disclosed that determines the electrical state at a workpiece disposed on an electrode in a plasma chamber that is in electrical communication with an RF signal source over a signal path. The signal path includes the electrode and the conductive path between the RF signal source and the workpiece. The method includes ascertaining an impedance of the signal path, sensing electrical characteristics associated with RF energy produced by the RF signal source and obtaining information on the electrical states at the workpiece. The electrical states include information concerning the RF voltage, RF current, the phase between the RF voltage and RF current, the DC bias voltage induced on the substrate, as well as a flux of ions striking the substrate. The electrical states are determined as a function of signal path impedance and the electrical properties of the RF signal. As a result, knowledge of the impedance of the signal path facilitates creating a real-time model of the electrical states at the workpiece by dynamically measuring the complete electrical characteristics, such as voltage, current, and phase difference, between the voltage and current of the RF signal, at some point along the signal path. To that end, the impedance may be determined by directly measuring the signal path employing an impedance meter or by calculating the same using well known physics and mathematical concepts. To provide a more accurate model of the electrical states at the workpiece, the modeling may include information concerning the impedance encountered by the RF signal that is introduced by the signal path. Accurate knowledge of the electrical states at the workpiece allows, inter alia, the completion of specific processing steps to be determined from temporal changes in the electrical state. This facilitates accurate control of the duration of plasma processing steps and affords process endpoint detection.
In accordance with an alternate embodiment of the method, the voltage, current and phase of the RF signal may be controlled by dynamically adjusting the RF generator in accordance with the modeling data or process endpoint from these measurements. In this manner, the characteristics of the RF signal may be maintained within predefined, or desired, parameters. A constant electrostatic force may be maintained between a pedestal and a workpiece electrostatically clamped to the pedestal, by adjusting the DC voltage supply in response to the determined electrical state.
The apparatus, in accordance with one embodiment, includes a processing chamber and a plasma generation system. The plasma generation system includes the electrode and the RF signal source. A sensor is in electrical communication with the electrode and the RF signal source. A controller is in electrical communication with the plasma generation system and a memory. The memory comprises a computer-readable medium having a computer-readable program embodied therein that carries out the functions of the methods.